Lecture notes Contexts of Care (SHN169)

SHN169 – Introduction to the Endocrine System


Introduction to the Endocrine System

Objectives of this Session
 Be able to explain what the endocrine system is, why it is needed and how it works
 Appreciate differences between endocrine & exocrine glands
 Explain what hormones are and what they do
 Understand the ways in which we classify hormones (functionally and chemically),
and be able to provide examples of each classification
 Appreciate what it means for a hormone to be water-soluble or lipid-soluble
 Understand the basics of steroid and non-steroid hormone action
 Name the 10 major endocrine organs/glands and understand the primary roles of
each
 Appreciate the consequences which can result when an endocrine organ/gland fails
to function correctly
 Understand the 3 main types of stimuli which serve to regulate the endocrine system

What is the Endocrine System?
 A system of specialised organs/glands throughout the body which secrete hormones
into the blood or extracellular fluid where they act as chemical messengers
o ‘Endo’ – means internal/within.
o ‘Crine’ – means to secrete
 Glands: organs which produce and secrete substances
 Intracellular fluid: the fluid inside a cell
 Extracellular fluid: the fluid outside a cell
 Chemical messenger: a compound which transmits a message
 Alongside the main 10, other organs secrete hormones as a secondary function: E.g.
Kidneys, heart, liver, stomach, skin
o Kidneys: produces erythropoietin in response to low blood oxygen levels
(hypoxia) → carried to bone marrow → stimulates stem cells → cells undergo
maturation – become red blood cells (erythrocytes)
o Heart: produces atrial natriuretic hormone → targets nephrons of kidneys →
promotes sodium (Na+ ) excretion → increased urine production & decreased
blood pressure

Why is it Needed? How does it Work?
 Functioning alongside the nervous system, the endocrine system is one of the key
systems in the maintenance of homeostasis
o Homeostasis: the maintenance of relatively constant states within the body
 Both the nervous and endocrine system use chemical messengers to achieve
maintenance of homeostasis
 Nervous system: uses neurotransmitters
 Endocrine system: uses hormones
 Maintenance of homeostasis by the endocrine system is achieved through hormone
action which serves to coordinate and regulate the functions of the systems of the
body



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,SHN169 – Introduction to the Endocrine System


o Some of the numerous aspects regulated include: organ-specific functions,
growth, metabolism, sleep, mood, reproduction, water and electrolyte
balance
 Hormones circulating within the blood come into contact with numerous cells
throughout the body
 Not all cells however are receptive to the hormones that come into contact with
them
o Target cells: cells possessing the receptors for a specific hormone and so are
receptive to their effects
o Target organ/tissue: organ/tissue containing the target cells for a specific
hormone
o Signal transduction: process by which the chemical signal is detected by
receptor on target cell
o Some cells have receptors for hormones (endocrine system) and
neurotransmitters (nervous system)
o Neurotransmitters
 Travel short distances using synapses
 Effects are rapid to appear but are short-lived
o Hormones
 Travel longer distances using the blood
 Effects are slower to appear but are long-lasting

Glands
 Organs which produce and secrete substances
 Two key types:
o Endocrine glands
 Secrete hormones
 Released directly into the blood / extracellular fluid
 They do not have ducts – referred to as ductless glands
 Either glandular epithelial cells or neurosecretory cells
 Neurosecretory cells
o Special, modified neurons (nerve cells)
o Secrete messenger molecules directly into bloodstream
o Secrete hormones rather than neurotransmitters
o Exocrine glands
 Secrete, e.g. sweat and tears
 Exo: external
 Release secretions out of the body via ducts
 Referred to as ducted glands
o Exocrine glands are not part of the endocrine system
o Endocrine glands are part of the endocrine system
 The 10 major Endocrine Organs/Glands




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, SHN169 – Introduction to the Endocrine System




Hormones
 Chemicals produced by glands or specialised cells of certain organs
 Secreted into the blood or extracellular fluid where they act as chemical messengers
 Hormones primarily serve to regulate the metabolic function/cellular activity of
other cells
 For a hormone to have an effect the cell must possess receptors for that specific
hormone
 Some common hormone effects:
o Alteration of plasma membrane permeability/electrical state
o Stimulation of protein/enzyme synthesis
o Activation/deactivation of enzymes
o Stimulation of secretions
o Stimulation of mitosis
 Many endocrine functions achieve maintenance of homeostasis through hormones
with opposing effects (e.g. insulin and glucagon in control of blood sugar)
 Functional classification of hormones
o Classification based on hormone function:
 Tropic hormones
 Target other endocrine glands
 Sex hormones
 Target reproductive tissues
 Anabolic hormones
 Bring about anabolism in their target cells
o i.e. hormones that build something up
o Chemical classification of hormones – i.e. Classification based on hormone
structure:




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